1. Field of the Invention
The present invention relates to a writing board ink composition which can be used to write on writing boards, such as the so-called whiteboard, with a marking pen, and which offers scrapable (erasable) writing.
2. Description of the Prior Art
In writing board marking inks which can be used to write on writing boards, such as the so-called whiteboard, with a marking pen, and which offer scrapable (erasable) writing, mixtures of ketone and ester solvents have traditionally been used as solvents. Also, various pigments fast to these solvents have been used to color such inks. For example, MAICROLITH RED BR-K and 4C-K of Ciba-Geigy AG, which are processed pigments prepared by dispersing a red azo pigment in a vinyl chloride-vinyl acetate copolymer resin carrier, can be used in writing board inks incorporating a mixed solvent of ketone solvent and ester solvent (Japanese Patent No. 936873).
In recent years, from the viewpoint of solvent odor and toxicity, there has been a need for the use of lower alcohol solvents. Therefore organic pigments fast to lower alcohol solvents are required. However, because conventional azo pigments are generally low in alcohol resistance when finely dispersed in lower alcohols, their use in writing board inks poses practical problems such as those in ink stability and writing erasability, since residual stain occurs upon scraping (erasing) the writing with an eraser etc. and increase in ink viscosity, pigment aggregation and other undesirable phenomena tend to occur during ink storage. Known organic pigments having excellent solvent resistance (alcohol solvent fastness), recognized as high quality pigments, include anthraquinone pigments (e.g., C. I. Pigment Red 168 and 167), condensed azo pigments (e.g., C. I. Pigment Red 144, 166 and 220), perillene pigments (e.g., C. I. Pigment Red 149, 178, 179 and 224). quinacridone pigments (e.g., C. I. Pigment Red 122, 207 and 209, and C. I. Pigment Violet 19), and thioindigo pigments (C. I. Pigment Red 38 and 88).
For these pigments to remain stable for a long time while in dispersion in an alcohol solvent in pens, they should be dispersed in the solvent in the form of ultrafine particles. However, this reduces the alcohol fastness of these pigments, which are recognized as good in alcohol fastness.
To ensure a stable dispersion system in such a state, resin is required to be used in an amount in proportion to the pigment surface area increased as the ultrafine particles are formed. However, increase in the resin content in a writing board ink incorporating a lower alcohol solvent poses problems, such as an increased occurrence of blurred writing, due to increase in ink viscosity, when the ink is used in pens, and a reduction in scrapability (erasability) in proportion to resin content.
Developed in view of the above problems in the prior art, the present invention is intended to provide a writing board ink composition incorporating a medium comprising, or based on, a lower alcohol solvent almost free of problems of odor and toxicity, wherein the pigment dispersed in the medium is fast to the above solvent despite its ultrafine size so that it remains stable for a long time while in pens even when the resin content is such that blurred writing and scrapability (erasability) reduction can be avoided.